Owing to their potential to harness the energies of both the singlet and triplet excitons after charge recombination, transition metal based phosphorescent materials have recently received considerable attention in fabricating organic light-emitting diodes (OLEDs). The main advantages are due to the heavy atom induced singlet-to-triplet intersystem crossing as well as the large enhancement of radiative decay rate from the resulting triplet manifolds. In this regard, numerous attempts have been made to exploit third-row transition metal complexes as dopant emitters for OLED fabrication, among which quite a few Pt(II), Os(II) and Ir(III) complexes have been reported to exhibit highly efficient device performances. Despite these developments, attempts to further expand the potential of the square planar Pt(II) complexes, in which the central metal ion possesses a higher atomic number than Os(II) and Ir(III) for efficient OLED applications, has encountered many intrinsic obstacles. For example, the PtOEP (H2OEP=octaethylporphyrin) type of emitter commonly has a ligand based phosphorescence with lifetimes as long as 30˜50 μs, so that saturation of emissive sites and a rapid drop in device efficiency at high drive current is observed. Also contributing to the poor device efficiency is the planar molecular configuration of many Pt(II) complexes, which leads to a stacking effect and hence the formation of aggregates or dimers that tend to form excimers in the electronically excited state. To recognize the potential of Pt(II) materials for applications in high efficiency OLEDs, the rational design of Pt(II) complexes aimed at reduction of the phosphorescence radiative lifetime and the prevention of stacking behaviour is critical.